In processing signals transported by data transmission and communication systems that use coaxial cable, it is often found that the signal is attenuated by the cable to such an extent that a correction is required to compensate the signal for the cable attenuation. Equalization of the transmission channels, which are often wire cables, is common in the field of data communication. The desired effect of equalization is to compensate for the high frequency loss of the cable so that the received waveform more closely resembles the transmitted waveform. This equalization reduces phase and amplitude distortions which otherwise can cause transmission errors. Fixed equalization may be used when the cable type and length are constant and known. Adjustable equalization may be used to manually accommodate differing cable lengths. Automatic equalizers determine the required equalization from the characteristics of the received signal, and then vary the applied equalization accordingly.
Many data transmission systems are bandwidth limited and appear as a simple low pass filter or phase-lag network which attenuates the higher frequency components in a data transmission. One way to compensate for this attenuation is to use a phase-lead or high pass filter in cascade with the data channel to "equalize" or flatten the overall attenuation characteristics of the system. Such conventional phase-lag and equalizer phase-lead networks are shown in FIGS. 1A and 1B, respectively. The basic equalizer of FIG. 1B, however, cannot provide the attenuation compensation and the desired impedance matching for lossy cables that have a complex attenuation vs. frequency characteristic.
A lossy cable typically is more complicated than a simple low pass filter, and accordingly, has an attenuation that varies with frequency in a more complicated manner than a simple low pass filter. In a lossy cable, the losses and resultant attenuation, .alpha., have a square root of frequency component, a.sub.1 .sqroot.f, and a linear component in frequency, a.sub.2 f; i.e., .alpha.=-(a.sub.1 .sqroot.f+a.sub.2 f) (db). Both a.sub.1 and a.sub.2 are obtained from a curve fit of the cable attenuation vs. frequency data. Level attenuation equalizers are typically used to compensate for lossy cables. FIG. 2 shows an attenuation vs. frequency curve for a typical level attenuation equalizer-cable system (line a), along with the attenuation vs. frequency curve for the cable alone (line b). Oftentimes, however, in a long cable data transmission system, level attenuation equalizers severely attenuate the overall signal, thereby providing unsatisfactory results.
One approach to the design of a conventional level attenuation equalizer is described in "The Design of Gigabit Copper Fibre Channel Equalized Cabling" by Sayre et al., and incorporated herein by reference, which describes a gigabit differential equalizer design methodology which, when coupled with appropriate skin effect cable transmission line models, permits the design and evaluation of gigabit copper fibre channel cabling. The equalizer design is based on a constant impedance differential bridged-H high pass filter. No compensation is included for circuit parasitics such as stray inductance, capacitance, or resistance.
Although the art of equalizers is well developed, there remain some problems inherent in this technology, particularly as increasingly higher frequency signals are being transported on data transmission and communication systems that use coaxial cable. Therefore, a need exists for a cable equalizer that compensates for attenuation in higher frequency signal transmission and that overcomes the drawbacks of the prior art.